This is a great question. The law here states that children have to be in school by age 7, which includes homeschooling or private school.
However, the schools tend to hassle people and don't tell them this information. Parents have to be vigilant about reading the laws.
My daughter started at 4 because her father insisted, and it was a mistake. I did not feel she was ready. But, we compromised and she went part-time.
But, my son has special needs. I homeschooled him and enrolled him in private school. When I went to the board of ed with his diagnosis and told them that he has special needs which the private school does not help with, they tried to say that he should have been enrolled in public school at age 4. But that's not what the law says. The person who said that to me is all pro-school and is very aggressive toward parents. I don't know how she keeps her job. (The school district gets $$ for each student because we live in a poverty-stricken area. So they get very aggressive with people who enroll their children elsewhere, such as private school. They lose the money that they would get if the child was enrolled at the public school.)